1. Field of the Invention
The present invention relates to a mold for molding a tire, which mold forms a tread pattern of a tire.
2. Description of the Related Art
In a conventional mold for molding a tire, it is easy for air to be sealed between the molding surface of the mold and the unvulcanized rubber at the time of vulcanization molding. In particular, because a pattern is formed at the molding surface of the mold by projections, the projections and the unvulcanized rubber close off escape paths which allow the air to escape, and air pockets are formed. As a result, recesses (bares) are formed in the surface of the molded tire due to the air pockets, thereby deteriorating the quality of the molded product.
Generally, a method of releasing the air to the exterior of the mold by providing minute holes (vent holes, microvents) in the mold is attempted. However, processes for forming the minute holes are required.
Further, when such vent holes or slits are formed, if the opening is too large, the rubber penetrates therein such that rubber projections are formed at the surface of the tire (so-called spewing), and a separate means for eliminating this drawback must be provided. In order to avoid providing a means for eliminating this drawback, the openings must be made small enough such that spewing does not occur (i.e., cannot be made any larger than a given limit), which is inconvenient.
As another method of venting air to the exterior of a mold, Japanese Patent Application Laid-Open (JP-A) No. 10-264169 discloses providing slits for air venting in portions of adjacent surfaces, which can be easily processed, of pieces of a mold.
FIG. 8 illustrates an example of a piece 01 of a mold disclosed in JP-A-10-264169.
The piece 01 has been formed by dividing a mold by radially-extending planes which pass through a central axis of the annular mold. Various projections 05, which provide the tire with its tread pattern, are formed at a molding surface 02 of the piece 01.
Slits 06 (the hatched portions in FIG. 8) are formed in portions of an adjacent surface 03 of the piece 01 so as to pass through from the molding surface 02 to the exterior surface.
However, these slits 06 of JP-A-10-264169 are formed in portions of the adjacent surface 03 of the piece 01. Thus, in the same way as when vent holes are used, a vacuum must be applied during vulcanization. Accordingly, costs of providing and costs of maintaining devices (a device for applying the vacuum, the mold structure adapted to the vacuum application, a vulcanization device adapted to sealing processing and the like) are incurred.
In the example illustrated in FIG. 8, the slits 06 are formed so as to penetrate through from the molding surface 02 to the exterior surface, and thus occupy a large surface area. Therefore, the surface area of the surface of the piece which abuts against the adjacent piece is small, the slits are relatively weak with respect to pressure such as thermal expansion at the time of vulcanization, and it is difficult to maintain the slits 06 over time.
The present invention was developed in order to overcome the above-described drawbacks, and an object of the present invention is to provide, at a low cost, a mold for molding a tire in which air can always be vented reliably without spewing occurring and without the need for applying a vacuum.
In order to achieve this object, a first aspect of the present invention is a mold for a tire, including: (a) a plurality of mold sections each having a pair of opposite side ends, a concavely curved molding surface extending between the side ends, and a recess defined by removing at a thin width the molding-surface-side portion of the outer surface of at least one of the side ends; and (b) a holder removably supporting the mold sections in an end-to-end assembly, wherein the mold sections cooperatively define an annular space for molding a tire, with the molding surface of each mold section facing a central location of the annular space, and each side end of the mold section facing a side end of an adjacent mold section.
In accordance with the first aspect, not portions of, but substantially all of the molding-surface-side side edge portion of the adjacent surface of the piece is cut-out at a narrow width (i.e., the molding-surface-side side edge portion is cut-out substantially along the entire width thereof). If a projection is provided at the adjacent surface, the portion of the side edge portion including the projection is cut-out as well. In this way, a gap for air venting which is continuous in an elongated manner is formed between adjacent pieces. Thus, air can be vented easily, and air can be vented reliably without applying a vacuum. Further, it is easy to prevent bares from being formed.
Furthermore, in accordance with the first aspect, although it is easy to vent the air, the width of the gap for air venting is sufficiently small, and thus, spewing can be reliably prevented.
Because the gaps are formed at the adjacent surfaces of the pieces, processing is easy. As described above, there is no need for providing a means or the like for applying a vacuum, and thus the costs of providing and maintaining the mold can be kept low.
The gaps for venting air are formed by cutting-out only the molding-surface-side side edge portions of the adjacent surfaces of the pieces. Thus, the surface area of the gap takes up a small proportion of the surface area of the adjacent surface, and the remaining, majority portion of the adjacent surface can be used as an abutting surface which abuts the adjacent piece. Accordingly, even if pressure such as thermal expansion is applied when the tire is vulcanized, gaps for air venting can be stably maintained and a reduction in the gaps over time can be kept to a minimum, as compared with conventional molds.
In a second aspect of the present invention, the width of the gap for air venting is from 0.005 to 0.05 mm.
When the width of the gap for air venting is from 0.005 to 0.05 mm, spewing does not occur, and there is no need for a means for eliminating spewing. Further, because the gaps are continuous in an elongated manner, there is no need to apply a vacuum during vulcanization.
In a third aspect of the present invention, the peripheral direction width of the piece is 15 to 50 mm.
When the peripheral direction width of the piece is 15 to 50 mm, the number of gaps for air venting can be sufficiently maintained in the entire mold, and air can be vented smoothly. Further, when the peripheral direction width falls in the range of 15 to 50 mm, there is no need to provide more pieces than necessary, and thus an increase in costs can be prevented.
In a fourth aspect of the present invention, a groove, which is wider than the gap, is formed at a deeper position than the gap (i.e., at the air flow downstream side of the gap) which gap is provided, as described above, by cutting-out the molding-surface-side side edge portion of the adjacent surface of the piece at a thin width.
In accordance with the fourth aspect, the wide groove is formed further toward the back than (at the downstream side of) the gap which is formed between the pieces, such that the cross-sectional surface area of the flow path for air is enlarged. Thus, the air venting effect can be improved even more.
In a fifth aspect of the present invention, the piece is divided along projections which are provided in the molding surface to form the tread pattern.
In accordance with the fifth aspect, because the piece is divided along the projections of the molding surface, the gaps for air venting of the adjacent surfaces are formed along the projections. At the time of vulcanization, the unvulcanized rubber material is set in close contact with the molding surfaces of the pieces, and usually, lastly comes into close contact with the base portions of the projections. Thus, by forming the gaps along the projections, air can be vented smoothly.
In a sixth aspect of the present invention, a small projection is formed in the molding surface of the piece, the small projection being oriented in the peripheral direction, and having a width of 0.1 to 0.3 mm and a height of 0.1 to 0.3 mm.
At the time of vulcanization, the unvulcanized rubber material lastly comes into close contact with the base portions of the small projections. Accordingly, air is reliably moved in the peripheral direction along the small projections, and is pushed out into the gaps of the adjacent surfaces of the pieces. Thus, an excellent air venting effect can be obtained.